The invention relates to a positively engaging clutch having a first driven clutch half that is connected to a first wheel drive train and is axially movable by an electronically actuated servo component, and a second driving clutch half which is connected to a main drive train and a second wheel drive train.
In heavy motor vehicles, in particular trucks or tractors, high torque forces must be transmitted, and this requires large and expensive components when friction clutches are used. In vehicles having a front wheel drive that can be engaged, and in vehicles having a locking intra-axle differential or a locking central differential, more compact positively engaging clutches, referred to herein as interengaging clutches, are frequently used in place of large friction clutches. An example of a positively engaging clutch is a dog clutch.
Interengaging clutches have, however, the great disadvantage that generally they cannot be shifted at all where the vehicle stands still, or can only be shifted by a very experienced driver during travel. Also, the two clutch halves of such a clutch must be brought into engagement with each other with a very fine touch and when the difference in RPM's is small. Generally, this must be done even before the vehicle reaches difficult terrain where a switch to four-wheel drive is required or where the differential lock would take effect, so that tire wear is increased. Disengagement is also difficult because considerable frictional forces arise between the clutch halves due to the torque between the claws of the clutch halves, and such frictional forces must be overcome when the clutch halves are to be pulled apart. Often a spring is used to decrease the expenditure of force needed to disengage the clutch; but this increases the force required for clutch engagement. For this reason, all-wheel drive or a differential lock is often allowed to remain engaged longer than necessary and is sometimes forgotten, leading to steering difficulties in normal road travel.
Hydraulic or pneumatic force amplifiers, or even electrical elements, are sometimes used as servo components to engage and disengage the clutch halves of an interengaging clutch, such as a dog clutch. Such automatic engagement systems are known, but they are expensive because of the necessity for a fine touch to engage the clutch.
The servo components in such systems must be bidirectional and must be of relatively large size as they must engage the clutch, keep it engaged during travel, and disengage it at the correct time. This requires a high energy consumption with hydraulic or pneumatic actuation because of leakages making it necessary for the oil or air pump to run constantly, and furthermore involving thermal losses with electric actuators.
DE-OS 40 21 653 discloses that the contact surfaces of a dog clutch can be inclined In the axial direction to decrease the required actuation forces. But this causes considerable and permanent axial forces to develop which must be absorbed by a locking device which increases the number of parts and requires more construction space. Yet a holding force, even if it is a small one, is still required, and although the torque opens the clutch automatically, it does so only after a disengagement force is applied.
However, if the engagement of the clutch is to be entirely automatic a second problem arises. This problem is that when the vehicle is not at a standstill, the slipping of the driven wheels as far as the vehicle is concerned, and the rotational speed difference between the two clutch halves (referred to herein as slippage) as far as the clutch is concerned, are the determining criteria for engagement of the clutch. On the other hand, the transmitted torque is the criterion for disengagement of the clutch.
When a dog clutch is engaged, the affected drive trains are rigidly connected so that no speed difference between the clutch halves is available as a criterion for disengagement. To measure torque, however, is not only expensive but much too imprecise to ensure disengagement at the correct moment. Automatic disengagement is also problematic because it must take place against the friction developed between the clutch surfaces being pressed together by the transmitted torque. In addition, measurement of the transmitted torque is also difficult because transient oscillations due to elasticity in the drive trains occur, so that defects in driving are not uncommon in practice.
The disadvantages and problems described above also apply when interengaging clutches are used in conjunction with locking intra-axle and inter-axle differentials such as are used in more expensive power drive transmissions.
It is the object of the instant invention to provide an improved interengaging clutch so that the problems described above can be avoided in a simple manner, i.e., to provide an interengaging clutch, such as a dog clutch which engages and disengages automatically and at the right moment.